The composite material, which is produced by sintering metal particles and ceramic powder together, has the high toughness originating from the metal and the high hardness and the high strength originating from the ceramics. Such a composite material is widely used in a variety of fields. For example, the tungsten carbide-cobalt system cemented carbide obtained by sintering tungsten carbide and cobalt, and the titanium carbide system cermet obtained by sintering titanium carbide and molybdenum are adopted as the edge for the cutting tool. Such a composite material is further blended, for example, with niobium carbide in some cases.
The composite material usually has a sufficient hardness. However, the composite material is sometimes required to have a higher hardness depending on the use or application. Then, a composite material is sometimes produced to have a shape depending on the use or application such that the composite material contains diamond or ceramics having higher hardness such as tetragonal system boron nitride (c-BN).
However, the composite material containing diamond or c-BN is not excellent in oxidation resistance. Further, the composite material containing diamond or c-BN is expensive. Therefore, in many cases, the surface of the composite material is coated with a thin film composed of a high hardness substance such as TiC and TiN, by means of the physical vapor deposition (PVD) method or the chemical vapor deposition (CVD) method.
However, when the thin film is formed by means of the PVD method or the CVD method, it is impossible to efficiently form the thin film, because the reaction efficiency is low and the reaction velocity is slow. For this reason, the production cost of the composite material becomes expensive. Further, the size and the shape of a workpiece are restricted, because the reaction chamber of the PVD apparatus or the CVD apparatus has a predetermined volume. Furthermore, the thin film is readily peeled off under a high stress.
As described above, if it is intended to realize a high hardness of the composite material, then the composite material is chemically unstable, and the production cost is expensive.